The present invention generally relates to variable speed drive motor systems and more particularly relates to variable speed drive motor systems for driving centrifugal compressors in refrigeration systems.
Operating efficiency of certain machinery depends on output shaft speed of a motor used to drive the machinery. The operating efficiency of such machinery may be optimized by proper control of the output shaft speed of the motor. For example, in a refrigeration system having a centrifugal compressor driven by an alternating current (AC) induction motor, operating efficiency of the compresser depends on rotational speed of the compressor as a function of compressor flow requirements (load on the compressor) and compressor lift requirements (pressure difference across the compressor). Thus, operating efficiency of the centrifugal compressor may be optimized by adjusting the output shaft speed of the AC induction motor driving the compressor, to properly match the rotational speed of the compressor to the compressor flow requirements and the compressor lift requirements as a function of time.
Control systems are known for continuously varying the output shaft speed of an AC induction motor, such as an AC induction motor used to drive a hermetic centrifugal compressor in a refrigeration system. These control systems comprise an inverter which directly regulates AC induction motor output shaft speed by controlling frequency and magnitude of electrical voltage signals supplied to electrical windings of the motor. In general, the complexity, size, and cost of such an inverter based control system depends on the electrical power requirements of the AC induction motor. Thus, for AC induction motors having relatively high electrical power requirements, which is typically the situation for AC induction motors driving hermetic centrifugal compressors in refrigeration systems, an inverter based control system is quite complex, relatively large, and very costly. Also, a major disadvantage of such an inverter based control system is that inefficiencies in the inverter plus increased inefficiency in the regulated motor due to inverter control combine to significantly reduce overall drive system efficiency.
Another known kind of control system for varying output drive shaft speed is a control system comprising an epicyclic gear system interconnecting a superimposing electric motor/generator with a main AC induction drive motor. However, this kind of control system is only capable of varying the output drive shaft speed in discrete steps and requires electro-mechanical contactors for switching the superimposing electric motor/generator between its generator function and its motor function. These electro-mechanical contactors and their associated hardware are relatively complex and expensive. Also, this kind of control system typically provides only a few different operating speeds, such as, for example, four operating speeds, and speed changes are abrupt. Also, the rapid, open transition switching of the electromechanical contactors in such a control system creates significant current and torque transients which may have a detrimental effect on the superimposing electric motor/generator and the epicyclic gear system.
In addition to the foregoing, a large number of other systems are known for providing a variable speed output drive. These systems may be generally classified into two categories. One category may be broadly defined as fixed speed inputs which are combined through a mechanical, fluid, or electrical interposing variable speed component. The other category may be broadly defined as various types of variable speed motors. However, these systems are not suitable for certain applications, such as for use in driving a centrifugal compressor in a refrigeration system, for one or more of the following reasons. First, many of these systems are not capable of changing speed while under load. Second, many of these systems are very large, complex and expensive. Third, many of these systems are very inefficient in operation. Fourth, many of these systems are incompatible with certain operating environments such as a refrigerant environment. Lastly, many of these systems present unsatisfactory reliability and service problems.